This invention relates to devices for the control of a fluid flow.
Devices intended to control the flow rate of a continuous flow of fluid are known in the form of needle valves. Although such valves are widely used they have inherent problems because they operate at small fixed clearances. Not only can this complicate any initial adjustment, but it also leaves them sensitive to fluid pressure variations. In addition, the small clearances at which the valves must usually be set can easily become blocked by dirt or contamination.
In GB 939661 a control valve is disclosed which has variable and fixed orifices in series in the fluid flow path through the valve. The fixed orifice establishes a pressure differential between chambers on opposite sides of a piston. A spring acts on the piston to hold it in equilibrium against the pressure differential. Variations in pressure at the inlet or outlet of the device will tend to alter the pressure differential and displace the piston. Piston movement adjusts the variable orifice in a sense that restores the original pressure differential. As a result, the flow through the valve is kept substantially constant. It will be noted, however, that this device still relies on a fixed orifice and so is also prone to disturbance due to dirt or contamination. An analogous arrangement is described in U.S. Pat. No. 4,513,777.